Marine structure with hydraulic tensioner

ABSTRACT

A tension leg marine structure or working platform is floatably positioned above an offshore working site, being maintained in place by a plurality of tension cables that connect to anchors at the sea floor. A riser which extends between the structure and the sea floor is laterally supported by a hydraulic tensioning system adapted to be adjusted in conformance with the platform&#39;s and the riser&#39;s movement.

BACKGROUND OF THE INVENTION

In the exploration and drilling for offshore oil and gas wells, one formof marine structure found to be desirable and effective is the tensionleg platform. In this type unit, the working structure is floatablysupported by its own buoyancy. However, tension cables applied to thelower end of the platform and fixed to the ocean floor, allow it to bedrawn downwardly to the desired working depth.

Under such conditions the structure is less susceptible to naturalforces such as wind and waves which would otherwise tend to displace anddisturb the horizontal orientation of the platform with respect to theocean floor.

In the drilling of offshore wells, it is necessary to utilize a riser,often referred to as a marine riser which extends from the well head tothe working deck of the floating platform. The riser member is in effectan elongated enclosure which surrounds and protects the drill string aswell as pipes which pass from the well upwardly to the platform deck.

Such risers are necessary for normal drilling operations but aresusceptible to damage and in many cases to breakage. The latter resultsfrom excessive strain applied to the riser as the floating platformvacillates about its working position in response to excessive wind andwave conditions at the water's surface.

Further, the riser is subjected to a considerable stress induced bywater currents and the like which pass around the riser, but which arenot particularly effective against the platform. In such an instance thenormally vertical riser disposition tends to be distorted as the latteris displaced laterally in one or more directions in response tounderwater currents.

In the presently disclosed arrangement, the elongated riser of the typecontemplated is provided with means directly attached to the tension legplatform such that the disposition and distortion of the riser isreadily controlled. The control means includes an adjustable connectionwhich extends from the centrally placed riser, outwardly toward therespective tension or hold-down cables. Thus, as the riser is subjectedto deflecting forces the connection is adjusted either automatically oras required to regulate the disposition of the riser.

It is therefore an object of the invention to provide means forsupporting a marine riser from a floating tension leg platformpositioned at an offshore body of water. A furtherobject is to avoiddamage to such a marine riserr which would occur if the riser were to beexcessively stressed as a result of underwater currents and forcesacting thereagainst. Another objective is to provide means forregulating the disposition and alignment of a marine riser throughsuitable adjustment thereof in a controlled manner from the deck of thefloating platform.

Towardachieving the above objectives and toward overcoming the hereinmentioned problems, the invention is directed to a riser connectionsystem adapted to cooperate with a floating, tension leg type marinestructure. The system forms an adjustable interconnection between thestructure and the marine riser. The riser connection system includesmeans for regulating the resisting forces applied to a riser as thelatter is deflected from its desired upright disposition.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view of the present offshore platform show in thesubmerged position being anchored to the ocean floor.

FIG. 2 is an alternate illustration of FIG. 1 showing the platform in adisplaced position.

FIG. 3 is an enlarged cross sectional view taken along line 3--3 in FIG.1.

FIG. 4 is a side elevation of FIG. 3.

Referring to FIG. 1, a floating tension leg platform 10 or marinestructure of the type contemplated is found to be both practical anddesirable for offshore drilling, exploratory and storage work. Theplatform embodies the advantage of being less susceptible to disruptionforces caused by wind and surface wave conditions, thus assuringcontinuous drilling operations in spite of the weather.

Functionally, platform 10 includes a working deck 11 which is normallypositioned 50 to 60 feet above the water's surface. A plurality ofdownwardly extending controllably buoyant members or legs 12, 13 and 14are supportably connected at their upper ends to the deck 11 therebymaintaining the unit at a desired level in the water. The respectivebuoyant members are normally of sufficient tank capacity that deck 11can be raised or lowered as desired through the use of a suitablecontrol system, for either transporting the structure, or forpositioning it at a working site.

While not shown in detail, the respective buoyant members are furtherprovided with cross bars 16 and other necessary understructure torigidize the unit.

To provide the necessary stability, a plurality of anchors 17, 18 and19, normally a minimum of three, are disposed at the ocean floor aboutthe site where a well 21 is to be drilled. These anchors are preferablypositioned to be directly beneath the platform 10 and are arrangedperipherally about the proposed well head.

Each anchor, such as 17, is firmly embedded into the ocean floor bypiling in the usual manner. However, if the anchor is of sufficientweight it can maintain its position in spite of upward forces actingthereagainst.

Anchor 17 is provided at its upper surface with cable holding rings orthe like such as cable directing members or pulleys through which ahold-down cable 23 is wound. In any instance anchor 17 is adapted toreceive a plurality of the tension members 23 which can be in the formof individual steel cables. The latter of course extend through thewater between the anchor 17 and floating structure 10.

As mentioned, each floor positioned anchor is preferably provided with aplurality of the hold-down cables 23. In the instant embodiment, atleast four are shown extending from each anchor 17, upwardly to deck 11.The respective cable ends are connected to a cable take-up mechanism 20on deck 11, which is adapted to adjust the cable tension and thusregulate the floating position of the structure. Ideally, the cablesshould extend in a substantially vertical direction between their upperand lower points of connection. While this disposition is preferred, itsabsence would not preclude use of the present riser control system. Forexample, the respective cables 23 can be canted to one side bydisplacement of the platform, or the anchors, and still serve thefunction for which they were designed.

Functionally, and prior to a well drilling operation, at such time asmarine structure 10 is positioned above a drilling site it is buoyed toa desired level by regulation of the respective legs 12, 13 and 14.Thereafter, the respective cables 23 are attached to preplaced anchors17, 18 and 19 at the ocean floor, and are reeled in or tensioned suchthat platform 10 is uniformly drawn deeper into the water although beingbuoyed upwardly by the same forces applied through the respective legs.At a predetermined working depth, the cable tensioning operation isterminated.

As presently positioned, and referring to FIG. 2, platform or structure10 is in effect tethered such that it can experience limited horizontalmovement. Such movement will embody a degree of vertical displacement.Thus, when subjected to a displacing force, the floating structure willbe moved laterally in an amount permitted by restraining cables 23, andcontingent on the platform's buoying force.

Normally, after platform 10 is positioned at a working site, the basicwell head equipment is installed. The latter is preferably embeddedbeneath the marine structure 10 such that a drill string, supported fromplatform deck 11, can be readily inserted and reinserted into the wellhead for a drilling operation. As presently shown, the normal well headequipment includes a base member 26 through which a number of casingmembers 27 are passed and supported. The usual Christmas treearrangement and control member, although not shown, are also included inthe base to regulate the drilling operation insofar as fluid flow isconcerned.

Base 26 is further provided with a suitable connector adapted tooperably engage the lower end of a drilling riser 28 at a removableconnector coupling.

Riser member 28 comprises an elongated tubular unit of sufficientdiameter to permit at least a drill string to pass downwardlytherethrough and to conduct drilling fluid from the well to the platformdeck 11. Normally riser 28 is fabricated of a series of short tubularsteel members which are end welded or bolted during installation to forma single continuous length. the riser is terminally fastened in suchmanner at both the well head and at the drilling deck 11, such that adrill string is readily inserted thereinto to perform a drillingoperation.

As presently shown, the lower end of riser 28 is pivotally connected atthe well head to allow a limited amount of relative movementtherebetween as platform 10 is displaced from its desired location abovethe well head.

As herein mentioned, a considerable amount of difficulty has beenexperienced with floating offshore structures of the type contemplatedparticularly in the use of such structures in deep water. Notably, suchdifficulties are prompted when platform 10 is displaced excessively, orwhen it is subjected to extreme movement due to weather conditions onthe water's surface. Thus, and as herein noted, riser 28 is susceptibleto both damage and to breakage.

To lessen the strain on riser 28 during a drilling operation, and toforce the riser into a desired configuration, the present riser systemis employed. The latter includes primarily adjusting means forconnecting centrally positioned riser 28 to the respective tensioningcables 23, located radially therefrom. The respective hold-down cableswill thus function to resist movement of the riser in such manner toabsorb any sudden shock or excessive stress which might otherwise beimposed.

Riser 28 is provided with a collar or ring 29 which firmly engages andencircles the riser 28 outer surface. While but a single ring 29 ispresently shown it is understood that a series of such rings or evenappropriately positioned fastening lugs can be applied to the riserexterior. Placement of collar 29 is normally done during the riserplacement period such that the riser adjusting system will act along aseries of longitudinally spaced stabilizing levels. In the instantembodiment, collar 29 includes a plurality of outwardly protrudingfastening lugs 31 which are peripherally spaced one from the other topermit the desired number of riser adjusting members 32 to beincorporated into the structure.

Each mounting lug 31 includes a center opening adapted to receive aconnecting pin 33 in such a manner to permit movement of hydrauliccylinder 32 in a plane normal to the riser 28 longitudinal axis.

The respective hydraulic cylinders 32 are shown as being operablypositioned at one extremity by collar 29, and extending radiallyoutwardly from the riser 28. The other extremity engages a mounting baseor carriage 34 disposed on the tensioning cables.

In the present embodiment the riser adjusting member 32 includes ahydraulic cylinder of the type generally utilized in the industry.Structurally it comprises an elongated hydraulic cylinder. A piston 36is slidably mounted within the cylinder to permit longitudinal motiontherewith. The piston further includes a piston rod which extends fromthe cylinder end.

Sealing means is carried on the cylinder to permit the piston to slidetherethrogh as a result of fluid being pumped into the cylinder underrealtively high pressure thereby affecting piston movement.

A hydraulic system is communicated with each hydraulic cylinder 32 andincludes a source of hydraulic fluid 37 such as a reservoir, which isnormally positioned above the water at deck 11. Fluid is conveyed to therespective cylinders 32 through individual lines or conduits 38 and 39to permit individual operation of each unit. Said lines 38 and 39 arepreferably supported at the riser 28 by spaced apart brackets 41 and maybe either a conduit or flexible. The lower ends of the respective fluidconduits are threadably engaged with the hydraulic cylinder at aterminal box 42.

In that the operation of hydraulic cylinders 32 presently shown followsthe basic function of similar such cylinders, and is well known in theart, a description of the workings of the cylinder is unnecessary.However, it is appreciated that the respective ends of the cylinder 32are separated by the piston 36 contained therein. Conduits 38 and 39extending to the cylinder are normally arranged to communicate withsuitable valving so that the hydraulic fluid is pumped into, or removedfrom the cylinder in accordance with the adjustment to be made on riser28. The remote end of the cylinder 32 includes piston rod 36 which isprovided with a fastening means adapted to operably engage a coupling 43on the vertically adjustable carriage 34. A picot pin 44 engages pistonrod 36 with carriage 34 at coupling 43 to allow movement of the cylinder32 to a desired disposition.

each set of tensioning cables 23 is provided with a carriage 34 at whichone end of hydraulic cylinder 32 is supportably mounted. Each carriage34 is vertically displaceable along the respective tensioning cables 23.The disposition of the hydraulic cylinder end is regulated so that thecylinder may be raised or lowered while being pivoted at the riseritself. this degree of movement affords a greater degree of versatilityto the overall system than would be realized were both ends of thehydraulic cylinders to be rigidly fixed.

each vertically traversing carriage 34 includes a plurality of pulleys46 and 47 mounted in tandem to a chassis 48 whereby to firmly fastenupon a hold-down cable 23. The respective pulleys are freely rotatableagainst and guided by the cable, the overall movement of the carriagebeing preferably regulated by means to be herein mentioned. Therespective carriage segments are fastened into the common chassis 48 bya cross brace 49. The latter includes an end plate depending therefrom,which in turn supports coupling 44 directed toward the centrallypositioned riser 28.

Hydraulic cylinder 32 is operably connected to the carriage 34 throughpin connection 44 such that the cylinder is free to be pivotally movedabout this connection as well as at the riser. The hydraulic cylindercan thus be raised or lowered at its remote end whereby to achievemaximum effectivenes thereof in acting against riser 28.

To fully utilize the versatility of carriage 34, it is preferablyoperably connected to a means for achieving the desired adjustment alongthe tensioning cables 23 in accordance with the configuration of theriser to be attained. In the present arrangement the control meansincludes an alignment cable 51 which is disposed coextensively with therespective pull-down cables 23. Said cable is further connected towind-up means 52 at deck 11 for progressing the cable 51 to achieve adesired disposition of hydraulic piston 32.

Alignment cable 51 comprises in effect a continuous member, having theends fastened to the respective upper and lower sides of carriage 34.Cable 51 is threaded through take-up mechanism 52 which is operable towind or unwind the cable thus to progress carriage 34 in an upward ordownward direction along cables 23.

Operationally, with tension leg platform 10 positioned for a normaloperation at the floor of a body of water, riser 28 will be subjected toa certain amount of non-vertical distortion and deformation in responseto the horizontal adjustment of the platform. Such horizontal movementoccurs in spite of anchors 17, 18 and 19 and in response to the variouswater currents acting against teriser.

There are a number of ways wherin the riser configuration can bedetermined from appropriate instrumentation. Thereafter, a program canbe determined for providing the riser with a desired configuration tobest promote drilling of a well. Such measuring equipment is known inthe petroleum industry and can provide a relatively accurate readingsuch that the exact configuration of te riser will be known.

since the tensioning cables 23 holding platform 10 beneath the water'ssurface are relatively straight and also canted from a normally verticalposition, a determination is made regarding the adjustment of eachindividual riser adjustment member 32 to act against the riser at aparticular angle.

With the riser configuration controlled, a drill string can be readilypassed therethrough for an operation such as straight or directionaldrilling. since, as mentioned, a plurality of rings 29, together withthe outwardly radiating hydraulic adjuster 32 can be utilized at spacedapart intervals along the riser, a series of such adjusters can beindividually utilized to achieve the desired pattern of te riser.

Other modifications and variations of the invention as hereinbefore setforth may be made without departing from the spirit and scope thereof,and therefor, only such limitations should be imposed as are indicatedin the appended claims.

I claim:
 1. In a tension leg marine structure for an offshore body ofwater, including means to apply a buoyant force to the structure,whereby to controllably regulate the floating disposition thereof in abody of water, anchor means at the floor of said body of water,including a plurality of anchors spaced about said floor in apredetermined pattern beneath said marine structure and tension cablemeans extending between said plurality of anchors and said structurerespectively, said cable means being adjustable by applying tensionthereto for establishing the floating disposition of said marinestructure in the water,a riser extending between, and conconnected atits respective extremities to said structure, and to a connector meansdisposed at the ocean floor, and a riser adjustment system including aplurality of hydraulic cylinders connected to and extending between saidtension cable means and said riser, said hydraulic cylinders having apiston operably contained therein, and control means including a sourceof hydraulic fluid communicating with said hydraulic cylinders, andvalve means operable to direct the flow of hydraulic fluid through saidhydraulic cylinders, whereby to adjustably regulate the horizontalspacing between said riser and said tension cable mean.
 2. In anapparatus as defined in claim 1, wherein said hydraulic cylinderincludes means at opposed ends thereof to operably e ngage said riser tosaid tension cable means respectively.
 3. In an apparatus as defined inclaim 1, including means to controllably regulate the angularrelationship between said riser and said hydraulic cylinder.
 4. In anapparatus as defined in claim 1, including vertical alignment meansdepending from said tension cable means and adapted to engage a remoteend of said hydraulic cylinder.
 5. In an apparatus as defined in claim4, including means for positioning said vertical alignment means adesired distance along said tension cables.
 6. In an apparatus asdefined in claim 1, including a carriage operably retained on saidtension cable means, and means communicated with said carriage forvertically regulating the relative position thereof with respect to saidriser.
 7. In an apparatus as defined in claim 1, wherein said verticalalignment means includes a cable connected to said carriage forcontrollably adjusting the vertical position thereof with respect tosaid collar.